Matching up perspective to background image
The reason you're having trouble is that your photo camera wasn't straight.  I can tell because the far corner isn't perfectly vertical.  It's VERY easy to fix this particular image:


go to the Camera view
Hit 'R' (Rotate)
Type -4.2 


Pretty darn close when you move it back to the corner.  You can tweak it a little either direction to make it more exact. Caveat is that I had to guess a bit because that gray floor was in the way since I just downloaded your example image.